The present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to conductive layers comprising carbon nanotubes and an improved capacitor comprising the improved conductive structure.
The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover all surfaces of the anode and serves as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials such as 7,7′,8,8′-tetracyanoquinonedimethane (TCNQ) complex salt, or intrinsically conductive polymers, such as polyaniline, polypyrol, polyethylenedioxythiophene and their derivatives. The solid cathode electrolyte is applied so that it covers all dielectric surfaces. An important feature of the solid cathode electrolyte is that it can be made more resistive by exposure to high temperatures. This feature allows the capacitor to heal leakage sites by Joule heating. In addition to the solid electrolyte the cathode of a solid electrolyte capacitor typically consists of several layers which are external to the anode body. In the case of surface mount constructions these layers typically include: a carbon layer; a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; a conductive adhesive layer such silver filled adhesive; and a highly conductive metal lead frame. The various layers connect the solid electrolyte to the outside circuit and also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
In the case of conductive polymer cathodes the conductive polymer is typically applied by either chemical oxidation polymerization, electrochemical oxidation polymerization or spray techniques with other less desirable techniques being reported.
The carbon layer serves as a chemical barrier between the solid electrolyte and the silver layer. Critical properties of the layer include adhesion to the underlying layer, wetting of the underlying layer, uniform coverage, penetration into the underlying layer, bulk conductivity, interfacial resistance, compatibility with silver layer, buildup, and mechanical properties. There has been a constant conflict in the art to optimize these various characteristics. For example, a higher concentration of resin is preferred for adhesion. As the resin concentration increases the adhesion of the carbon layer improves. Conductivity on the other hand occurs through the carbon particles and therefore it is preferred to minimize the resin to insure adequate conductivity. Those of skill in the art have heretofore been forced to optimize the conflicting parameters of adhesion with conductivity. It has long been considered important to avoid decreasing the carbon content due to the loss of conductivity.
U.S. Pat. No. 6,556,427 attempts to circumvent the conflict between adhesion and conductivity of the carbon layer by allowing the binder of the carbon paste to infiltrate into the solid electrolyte layer. Controlling the degree of infiltration is difficult and variability in the infiltration will alter the composition of the carbon layer thereby resulting in variability in conduction and in adhesion with a subsequent layer.
The resistance across the carbon layer increases as the carbon buildup increases since the electrical path length across the layer is increased. However, thin layers provide less thermo-mechanical protection to the dielectric. Therefore, the carbon layer has long been considered necessary and yet a limiting factor in the further advancement of solid electrolytic capacitors.
The silver layer serves to conduct current from the lead frame to the cathode and around the cathode to the sides not directly connected to the lead frame. The critical characteristics of this layer are high conductivity, adhesive strength to the carbon layer, wetting of the carbon layer, and the mechanical properties. Compatibility with the subsequent layers employed in the assembly and encapsulation of the capacitor are also critical. In the case where a silver adhesive is used to attach to a lead frame compatibility with the silver adhesive is an issue. In capacitors which utilize solder to connect to the external lead solderability and thermal stability are important factors. In order for the solder to wet the silver layer, the resin in the silver must degrade below the temperature at which the solder is applied. However, excessive degradation of the resin creates an effect termed “silver leeching” resulting in a poor connection between the external cathode layers and the external cathode lead. The traditional approach to applying a silver layer requires a delicate compromise in thermal stability of the resin in order to simultaneously achieve solder wetting and to avoid silver leeching.
Through diligent research the present inventors have developed a carbon layer which circumvents the problems encountered in the prior art.